Electrochemical processing of inorganic chemicals in electrolytic diaphragm cells for the production of other inorganic materials is well known. The electrolytic cell generally comprises an anolyte compartment containing an anode, a catholyte compartment containing a cathode, and a liquid electrolyte-permeable microporous diaphragm that separates the anolyte compartment from the catholyte compartment. Diaphragms are used, for example, to separate an oxidizing electrolyte from a reducing electrolyte, a concentrated electrolyte from a dilute electrolyte, or a basic electrolyte from an acidic electrolyte.
A non-limiting example of an electrolytic diaphragm cell is the electrolytic diaphragm cell used for the electrolysis of aqueous alkali metal halide solutions, e.g., a chlor-alkali cell. In such an electrolytic cell, the diaphragm is generally formed on a foraminous metal cathode (which together form a cathode structure) and separates an acidic liquid anolyte from an alkaline liquid catholyte. The electrolysis of an aqueous alkali metal halide solution (liquid brine) generally involves introducing liquid brine into the anolyte compartment of the cell and allowing the brine to percolate through the liquid brine-permeable microporous diaphragm into the catholyte compartment. The microporous diaphragm is sufficiently porous to allow the hydrodynamic flow of brine through it, while at the same time inhibiting the back migration of hydroxyl ions from the catholyte compartment into the anolyte compartment.
When direct current is applied across the electrodes of the alkali metal halide electrolytic cell, halogen gas is evolved at the anode, hydrogen gas is evolved at the cathode, and an aqueous alkali metal hydroxide solution is formed in the catholyte compartment. In the case of the electrolysis of aqueous sodium chloride solutions, the halogen produced is chlorine and the alkali metal hydroxide formed is sodium hydroxide. Catholyte liquor comprising alkali metal hydroxide and unconverted brine is removed from the catholyte compartment of the cell, and alkali metal hydroxide is recovered from the catholyte liquor.
In addition to the primary products (halogen and alkali metal hydroxide) that are produced during electrolysis of aqueous alkali metal halide solutions, small amounts of alkali metal chlorate, e.g., sodium chlorate, can be produced as a by-product. Alkali metal chlorate by-product is found in the recovered alkali metal hydroxide product. Alkali metal chlorate is corrosive to process equipment downstream of the electrolytic cell and its presence in the alkali metal hydroxide product may require treatment of that product to reduce or remove the alkali metal chlorate present therein, thereby to avoid its corrosive effect on downstream process equipment. The production of alkali metal chlorate may also result in a reduction in the efficiency of the electrolytic cell.